Blue milk. Blue milk has been known for many years, its communicable nature being established as long ago as 1838. It appears on the surface of milk first as isolated particles of bluish or grey color, which later become confluent, the blue color increasing in intensity as the acidity increases. The causal organism, B. cyanogenes, is very resistant toward drying,[75] thus accounting for its persistence. In Mecklenberg an outbreak of this sort once continued for several years. It has frequently been observed in Europe in the past, but is not now so often reported. Occasional outbreaks have been reported in this country.
Other kinds of colored milk. Two or three chromogenic forms producing still other colors have occasionally been found in milk. Adametz[76] discovered in a sample of cooked milk a peculiar form (Bacillus synxanthus) that produced a citron-yellow appearance which precipitated and finally rendered soluble the casein. Adametz, Conn, and List have described other species that confer tints of yellow on milk. Some of these are bright lemon, others orange, and some amber in color.
Still other color-producing bacteria, such as those that produce violet or green changes in the milk, have been observed. In fact, almost any of the chromogenic bacteria are able to produce their color changes in milk as it is such an excellent food medium. Under ordinary conditions, these do not gain access to milk in sufficient numbers so that they modify the appearance of it except in occasional instances.
Treatment of abnormal fermentations. If the taint is recognized as of bacterial origin (see p. 57) and is found in the mixed milk of the herd, it is necessary to ascertain, first, whether it is a general trouble, or restricted to one or more animals. This can sometimes be done by separating the milk of the different cows and noting whether any abnormal condition develops in the respective samples.
Fermentation tests. The most satisfactory way to detect the presence of the taints more often present is to make a fermentation test of one kind or another. These tests are most frequently used at the factory, to enable the maker to detect the presence of milk that is likely to prove unfit for use, especially in cheese making. They are based upon the principle that if milk is held at a moderately high temperature, the bacteria will develop rapidly. A number of different methods have been devised for this purpose. In Walther's lacto-fermentator samples of milk are simply allowed to stand in bottles or glass jars until they sour. They are examined at intervals of several hours. If the curdled milk is homogeneous and has a pure acid smell, the milk is regarded as all right. If it floats in a turbid serum, is full of gas or ragged holes, it is abnormal. As generally carried out, no attempt is made to have these vessels sterile. Gerber's test is a similar test that has been extensively employed in Switzerland. Sometimes a few drops of rennet are added to the milk so as to curdle the same, and thus permit of the more ready detection of the gas that is evolved.
Wisconsin curd test. The method of testing milk described below was devised at the Wisconsin Experiment Station in 1895 by Babcock, Russell and Decker.[77] It was used first in connection with experimental work on the influence of gas-generating bacteria in cheese making, but its applicability to the detection of all taints in milk produced by bacteria makes it a valuable test for abnormal fermentations in general.
In the curd test a small pat of curd is made in a glass jar from each sample of milk. These tests may be made in any receptacle that has been cleaned in boiling water, and to keep the temperature more nearly uniform these jars should be immersed in warm water, as in a wash tub or some other receptacle. When the milk is about 95° F., about ten drops of rennet extract are added to each sample and mixed thoroughly with the milk. The jars should then remain undisturbed until the milk is completely curdled; then the curd is cut into small pieces with a case knife and stirred to expel the whey. The whey should be poured off at frequent intervals until the curd mats. If the sample be kept at blood heat (98° F.) for six to eight hours, it will be ready to examine.
Fig. 18.
Improved bottles for making curd test. A, test bottle complete; B, bottle showing construction of cover; S, sieve to hold back the curd when bottle is inverted; C, outer cover with (D H) drain holes to permit of removal of whey.]